Using the threshold carrier density measurement technique, the differential gain dg/dn at lasing threshold was measured. It was found that the differential gain dg/dn of 1. 3- mu m InGaAsP lasers is a strong function of the active layer doping level P//0. At a doping level of 2. 5 multiplied by 10**1**8cm** minus **3, the differential gain is several times larger than at 4 multiplied by 10**1**7cm** minus **3. A factor of 2 increase in the modulation bandwidth is demonstrated using the dg/dn dependency on P//0.